Breakpoint chlorination and chlorine demand

Reviewed by the Pool Operator Prep editorial team. Worked from public water-chemistry and hydraulics facts. Last verified: 2026-07-24.

How do you calculate the chlorine needed to reach breakpoint?

Multiply the combined chlorine by about 10 to get the free-chlorine target, the standard superchlorination rule. Then dose to raise free chlorine to that target, not just to your normal residual. Combined chlorine of 0.5 ppm means shocking to roughly 5 ppm free chlorine, and stopping short only makes chloramines worse.

To pass breakpoint you raise free chlorine to roughly 10 times the combined chlorine, the standard operating rule. Chlorine demand is the amount consumed before a stable free residual holds, so you dose to the target, not just to the residual you want left over.

The formula, worked

Formula - free-chlorine target for breakpoint

Target FC ≈ 10 × combined chlorine (CC)

Worked example

Combined chlorine reads 0.5 ppm, current free chlorine 1.0 ppmBreakpoint target: 10 × 0.5 = 5.0 ppm FCRaise FC by: 5.0 − 1.0 = 4.0 ppm, then dose that change

= superchlorinate to 5.0 ppm FC (a 4.0 ppm raise)

Common slip Dosing only to the combined-chlorine value, or to your normal 2 to 3 ppm target, never reaches breakpoint. Stopping below breakpoint makes chloramines and the pool smell worse, not better. The 10-to-1 rule is a widely used operating heuristic; confirm the target against your jurisdiction's shock guidance.

A line that dips into a shallow valley marked with a coral dot before rising steeply, representing chlorine demand dropping to its lowest point at breakpoint before free chlorine rises. — Demand drops to its lowest point (the coral dot, breakpoint) right before free chlorine rises.

Where this shows up

Breakpoint sits on top of two other drills: combined chlorine tells you how far to go, and chemical dosing converts the free-chlorine raise into pounds of product. The practice test drills the disinfection knowledge behind breakpoint.

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Problem set: 6 worked problems

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Watch for To pass breakpoint you raise free chlorine to roughly ten times the combined chlorine, the standard superchlorination rule. Dosing only to your normal residual, or to the combined-chlorine value, never reaches breakpoint and makes chloramines worse. Confirm the shock target against your jurisdiction's guidance.

Combined chlorine reads 0.5 ppm. Using the ten-to-one rule, what free-chlorine level should you shock to?
- A About 0.5 ppm
- B About 15 ppm
- C About 3.0 ppm
- D About 5.0 ppm free chlorine
Show the worked solution

Correct answer: D. About 5.0 ppm free chlorine

Breakpoint target = about 10 x combined chlorine = 10 x 0.5 = 5.0 ppm free chlorine. Shocking only to 0.5 ppm (the combined value) or to a normal 3 ppm residual stops short of breakpoint, so the chloramines are not destroyed.

Source: Public operating rule: reach breakpoint at roughly 10 times the combined chlorine
Combined chlorine is 0.3 ppm and current free chlorine is 1.5 ppm. Using the ten-to-one rule, how much must you RAISE free chlorine to reach breakpoint?
- A About 3.0 ppm
- B About 0.3 ppm
- C About 30 ppm
- D About 1.5 ppm
Show the worked solution

Correct answer: D. About 1.5 ppm

Breakpoint target = 10 x 0.3 = 3.0 ppm. You already have 1.5 ppm, so the raise is 3.0 - 1.5 = 1.5 ppm. Dosing the full 3.0 ppm on top of the existing 1.5 would overshoot; the question asks for the raise, not the target.

Source: Public operating rule: breakpoint target is about 10 times combined chlorine, then dose the difference from current free chlorine
Combined chlorine 0.4 ppm, current free chlorine 2.0 ppm, pool 30,000 gallons. How much cal-hypo (65%) reaches breakpoint?
- A About 1.54 lb
- B About 0.50 lb
- C About 0.19 lb
- D About 0.77 lb
Show the worked solution

Correct answer: D. About 0.77 lb

Breakpoint target = 10 x 0.4 = 4.0 ppm; current is 2.0 ppm, so raise by 2.0 ppm. Pure weight = 30,000 x 2 x 8.34 / 1,000,000 = 0.50 lb; divide by 0.65 = 0.77 lb of cal-hypo. Dosing to the full 4.0 ppm from zero would need 1.54 lb.

Source: Public operating rule plus dosing relationship: target 10 x CC, then pure lb = gallons x ppm x 8.34 / 1,000,000 / fraction
Why is it a mistake to add just enough chlorine to match the combined-chlorine reading?
- A Combined chlorine is harmless, so no shock is needed
- B Free chlorine and combined chlorine are the same thing
- C It wastes chlorine with no downside
- D Below breakpoint the chlorine forms more chloramines and the pool smells worse
Show the worked solution

Correct answer: D. Below breakpoint the chlorine forms more chloramines and the pool smells worse

Chloramines are only destroyed once free chlorine climbs past the breakpoint, roughly ten times the combined chlorine. Adding a small amount short of breakpoint feeds the chloramine reaction, so the water gets more irritating and smells stronger, not cleaner.

Source: Public disinfection chemistry: chloramines are oxidized only above the breakpoint
Combined chlorine reads 0.6 ppm. What free-chlorine level should you shock to?
- A About 0.6 ppm
- B About 4.0 ppm
- C About 12 ppm
- D About 6.0 ppm free chlorine
Show the worked solution

Correct answer: D. About 6.0 ppm free chlorine

Target = 10 x 0.6 = 6.0 ppm free chlorine. The rule scales directly with the combined-chlorine reading, so a higher chloramine load means a higher shock target.

Source: Public operating rule: reach breakpoint at roughly 10 times the combined chlorine
Which reading most directly tells you a pool needs to be shocked to breakpoint?
- A Free chlorine above 3 ppm
- B Total alkalinity above 120 ppm
- C pH above 7.6
- D Combined chlorine at or above about 0.2 to 0.4 ppm
Show the worked solution

Correct answer: D. Combined chlorine at or above about 0.2 to 0.4 ppm

Combined chlorine is the chloramine load; once it reaches roughly 0.2 to 0.4 ppm it signals a shock to breakpoint. A high free chlorine, high pH, or high alkalinity are different problems with different fixes.

Source: Public operating guidance: shock when combined chlorine reaches about 0.2 to 0.4 ppm

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